1. Technical Field
The present invention relates to an apparatus for calculating a polarization voltage of a secondary battery and an apparatus for estimating a state of charge (SOC) of the secondary battery based on the calculated polarization voltage.
2. Background Art
Each of motor vehicles that is supplied with driving force from an electric motor, such as a pure electric vehicle and a hybrid electric vehicle, is equipped with a secondary battery. The electric motor is driven by electric power stored in the secondary battery. The motor vehicle has a function of letting the electric motor work as an electric generator during regenerative braking operation; namely, during braking of the vehicle and convert kinetic energy of the vehicle into electric energy, thereby activating a brake. The thus-converted electric energy is stored in the secondary battery and reused at the time of performance of acceleration or in a like situation.
When the secondary battery experiences overdischarging operation or overcharging operation, battery performance of the battery is deteriorated. For this reason, there is a necessity for regulating charging or discharging operation by grasping the state of charge (SOC) of the secondary battery. In particular, a hybrid electric vehicle often control the state of charge in such a way that the SOC comes into an approximate midrange (50% to 60%) between a fully charged state (100%) and a fully empty state (0%) in order to let the secondary battery accept regenerated power and make the secondary battery able to immediately feed electric power to the electric motor on request. In this case, more accurate detection nor estimation of the SOC is expected.
A disclosure of JP 10-319100 A includes: observing a battery current obtained in the course of driving operation and thereby estimating a degree of polarization by predicting a local change in concentration of an electrolyte; and also estimating the state of charge based on a voltage-current characteristic that has been measured by waiting for a chance of influence of polarization becoming low, by reference to a map of a correction factor and a voltage value appropriate to a state of charge.
A disclosure of JP 2000-258514 A includes taking into account a polarization electromotive force voltage which is included in electromotive force and which stems from polarization of a battery cell when an SOC is determined on the basis of whether or not the electromotive force voltage that has been obtained by subtracting, from a voltage, a voltage drop in a battery attributable to internal resistance of the battery has become equal to or greater/less than a predetermined value.
A disclosure of JP 2001-97150 A includes: controlling a generated output from an electric generator in such a way that an index of polarization in a battery becomes no less than a lower limit value and no greater than an upper limit value and estimating an SOC of the battery on the basis of such a polarization index.
A disclosure of JP 2003-68370 A includes performing more accurate estimated calculation of a polarizing electromotive voltage by taking into account mutually-different polarization characteristics of battery constituent members making up a battery.
A disclosure of JP 2003-197275 A includes: determining a polarization voltage, by reference to a reference table, based on an amount of change in filtered, integrated capacity; subtracting the polarization voltage from an effective no-load voltage, to thus determine electromotive force of a battery; and estimating an SOC based on the battery electromotive force by reference to the reference table.
A disclosure of JP 2008-180692 A includes: subtracting a polarization voltage from a no-load voltage, to thus determine electromotive force; making a correction to the electromotive force calculated this time in such a way that an amount of difference between a previously-calculated electromotive force and the electromotive force calculated this time does not exceed a predetermined limiting value; and estimating an SOC based on the thus-corrected electromotive force.
As mentioned above, it is possible to calculate a no-load voltage of the secondary battery; determine electromotive force of the secondary battery by means of subtracting a polarization voltage from the no-load voltage, and estimate the state of charge (SOC) of the secondary battery based on the thus-calculated electromotive force of the secondary battery. However, as is obvious from the above calculation processes, the accuracy of calculation of the polarization voltage comes to exert great influence on the accuracy of calculation of the SOC. A disclosure of JP 2001-97150 A includes controlling an electric generator in such a way that a polarization index falls within a range between the lower limit value and the upper limit value and estimating an SOC under the condition. A characteristic of the secondary battery can change according to a temperature, or the like. There is no disclosure of such a characteristic of the secondary battery being taken into account at the time of calculation of a polarization voltage. Further, when a polarization voltage is calculated based on an electric current flowing through the secondary battery, it may be the case where an erroneous current value may be acquired because of an error in a measured current value itself or a lag in measurement timing. If an SOC is calculated based on the polarization voltage calculated based on the erroneous current value, there will arise a case where a secondary battery will undergo overcharging or overdischarging.